Electrostatic air filters have been known in the art for many years. Some of the earliest electrostatic air filters have configurations in which the filtering media accumulates a charge by virtue of air passing through that media. One such apparatus which develops an electrostatic charge from moving air used a mat made of filaments or fibers of polyethylene (See U.S. Pat. No. 2,612,966). A similar device is taught by U.S. Pat. No. 4,229,187, wherein a polymeric material becomes self-charged in the presence of moving air, using such preferred materials as polyester, nylon, and polypropylene.
Because the efficiency of self charging filters is low, the majority of electrostatic air filters use some type of high-voltage source electrically connected to the filter media, and/or a similar high-voltage source electrically connected to an electrode which is used to ionize particles in the air that are then collected by a filter media. One early such electrostatic air filter uses wires or rods to impart a charge on a paper filter element, preferably using a paper having a high rayon content (See, U.S. Pat. No. 2,814,355). Another design discloses the use of filter batts made of metallic and dielectric filamentary materials, such as DYNEL.TM. and fine aluminum filaments, wherein the electrical charge is transferred to the filter batt by direct electrical connection to a high-voltage source (See, U.S. Pat. No. 3,053,028). A further configuration using filter batts teaches the use of any suitable medium, such as glass fibers, which is capable of being electrostatically charged to attract and hold particles (See, U.S. Pat. No. 3,105,750).
Other configurations of electrostatically-charged filters have been taught in the prior art, including an electrostatic filter panel made of a charged cotton mesh pad having a conductive coating (See, U.S. Pat. No. 3,073,094). Another design using a dielectric filter material such as a polyester media is taught in U.S. Pat. No. 3,763,633. This reference also teaches a wire screen grid sandwiched between two open cell polyurethane foam filters, and additionally teaches that as the filter cell becomes dirtier, it also becomes more efficient in removing particulates. Another design teaching the use of open cell foam polyurethane as the filtering media is set forth in U.S. Pat. No. 4,115,082.
A further configuration of electrostatic air filters is taught in U.S. Pat. No. 3,910,779, in which the filter is in a bag configuration made of cloth or other textile fabric. A yet further configuration of electrostatic air filters is taught in U.S. Pat. No. 4,185,972, in which an electret filter media contains a built-in charge. In the preferred embodiment of this reference, the filter consists of polypropylene fibers coated with a metallic coating. Another electrostatic air filter design, set forth in U.S. Pat. No. 4,781,736, discloses the use of non-conductive fibrous filter sheets made of fiberglass, which are incorporated within an electric field formed between spacers. The charge is induced on the filter elements by electrodes, rather than by action of charged particles themselves. Another design, disclosed in U.S. Pat. No. 4,978,372, has a pleated charged media consisting of a fibrous filter pad which is disposed between adjacent pairs of charging media. The preferred embodiment of this reference discloses a pad made of fiberglass. However, other dielectric fibers such as polyester and blends of polyester and cotton can also be used.
A further electrostatic air filter design in disclosed in Canadian Patent No. 1,272,453). This Canadian patent provides a disposable rectangular "cartridge" which is connected to a high voltage power supply. The "cartridge" consists of a conductive inner screen which is sandwiched by two layers of a dielectric "fibrous material" (either plastic or glass), which, in turn, is further sandwiched by two outer screen layers of conductive material. The conductive inner screen is raised to a high voltage via an electrical connection to the high voltage power supply, thereby imparting an electrostatic field across the two dielectric layers.
A major failing of the prior art is that the foam or fibrous filter materials disclosed in the past tended to pick up moisture from humid air traveling therethrough. When the foam or fibrous media accumulated such moisture, the electrostatic charge (i.e., the electric field) tended to be dissipated through the moist media, thus gradually making the electrostatic filters ineffective. Because the water vapor in the atmosphere eventually is absorbed in the charged media used in existing electrostatic air filters, the prior art has not disclosed a method or apparatus which can properly work in humid environments.
Another failing of the prior art has been the inability to achieve good efficiency at velocities that are economically practical for most applications.
Cartridge-type filters, which are cylindrically-shaped air filters that have "dirty" air directed through an outer layer of filter media and have "clean" air directed out of the center of the cylinder, are typically used in dust collecting systems. A further failing of the prior art, particularly in the use of such cartridge-type filters, is that the media used in the filter does not achieve its nominal efficiency (a later, higher efficiency than its initial efficiency) until after a certain coating of particulates has been accumulated. In applications like filtration of inlet air to gas turbines used to generate electricity, the cartridge filter will operate for weeks at low efficiency for small particle size removal. In fact, many manufacturers of cartridge filters use limestone dust or some other substance to coat such filters in order to achieve their nominal efficiency at the time the filters are first used. A disadvantage of this coating process is that the "new" filter is already partially used up, and therefore, is deprived of a certain amount of its useful lifetime before it becomes completely clogged due to its increased differential pressure drop as air moves across.